With the recent advanced semiconductor technique, there are increasing tendencies to have fine structures of wiring etc. for semiconductor devices. However, when the finer the circuit of the wiring etc. of a semiconductor device, the higher possibility that an α ray and a cosmic ray (neutron beam) which are externally received incur an operation error of the circuit. As for memory, a small memory unit of a large capacity has been produced. However, since the structure of the circuit is fine, there is the possibility that the α ray and the cosmic ray (neutron beam) cause an error in the stored data. The error is called a soft error. The soft error can be caused in both main memory and cache memory.
The write system for the cache memory can be a write back system and a write through system. It is known that although the operation speed performance of the write back system is higher but the control of the write back system is more complicated. However, as for a soft error, the write through system is recommendable in view of a soft error because in the write back system the data contents stored in the cache memory are often written back to the main memory after a lapse of a long time, and the data as the memory contents can be inverted, thereby causing a conspicuous soft error when the data is written back. On the other hand, in the write through system, data is written to the cache and simultaneously to the main memory, thereby reducing the operation of reading the contents after a lapse of a long time, and allowing the soft error rate to be apparently lower.
Therefore, it is effective that the write through system is adopted as a cache memory system at the cost of the operation speed performance of the cache memory.
When the write through system is used for cache memory, it is said that the same data contents can be maintained between the cache memory and the main memory. based on this, when an error occurs in the cache memory, the data in the cache memory is discarded, the data contents in the main memory corresponding to the address in the cache where the error has occurred can be read and transmitted to the CPU, and the data contents in the cache memory where the error has occurred can be rewritten, thereby resuming a normal operation.
The conventional technology includes correcting an error as a temporary error in buffer memory, preventing an error exceeding the number of correctable bits by a combination of a soft error by an α ray and an error from a hardware fault, and suppressing the accumulation of soft errors.